GH pandas-dev#24845

pandas/core/indexes/category.py

@@ -324,7 +324,9 @@ def _format_attrs(self):
                 "categories",
                 ibase.default_pprint(self.categories, max_seq_items=max_categories),
             ),
-            ("ordered", self.ordered),
+            # pandas\core\indexes\category.py:315: error: "CategoricalIndex"
+            # has no attribute "ordered"  [attr-defined]
+            ("ordered", self.ordered),  # type: ignore[attr-defined]
         ]
         if self.name is not None:
             attrs.append(("name", ibase.default_pprint(self.name)))
@@ -659,6 +661,14 @@ def map(self, mapper):
 
     def _concat(self, to_concat: List["Index"], name: Label) -> "CategoricalIndex":
         # if calling index is category, don't check dtype of others
+        count = 0
+        for c in to_concat:
+            if not to_concat[0].ordered:
+                if count != 0:
+                    to_concat[count] = CategoricalIndex(
+                        list(to_concat[count].values), list(to_concat[0].categories))
+                # c.categories = ['a', 'b']
+            count += 1
         codes = np.concatenate([self._is_dtype_compat(c).codes for c in to_concat])
         cat = self._data._from_backing_data(codes)
         return type(self)._simple_new(cat, name=name)

test_issue1.py

@@ -0,0 +1,17 @@
+import pandas as pd
+
+
+cat1 = pd.CategoricalIndex(
+    ["a", "a",],
+    categories=["a", "b"],
+    ordered=False)
+cat2 = pd.CategoricalIndex(
+    ["b", "b"],
+    categories=["b", "a"],
+    ordered=False)
+
+df1 = pd.DataFrame({"A": [1, 2]}, index=cat1)
+df2 = pd.DataFrame({"A": [3, 4]}, index=cat2)
+#print(df1)
+#print(df2)
+print(pd.concat((df1, df2)))  # cat2 disappeared from index